This invention relates to printers for computer systems and more particularly to Cathode Ray Tube (CRT) exposure systems within such printers. Even more particularly, the invention relates to using a graded index lens array to conduct light from the surface of a CRT to expose photosensitive media.
Digital photographic process printers that utilize cathode ray tubes (CRT""s) for media exposure typically expose the photographic media line by line directly from the face of the CRT or through fiber optics. That is, the light from each element, or pixel, on the face of the CRT is conducted to the media directly, through a conventional lens, or through a fiber optic face plate. The interior surface of the face plate typically has three phosphor stripes or bands; one for red, one for green, and one for blue, for the three color components that make up the elements. The controller for the CRT uses linear deflection amplifiers to select a phosphor and sweep an electron beam across one phosphor on the face of the CRT to expose one line on the media to one color at a time. This exposes the media in a deflection cycle that typically consists of a sweep, retrace, and hold period. During the sweep period, the CRT electron beam is unblanked and the beam is driven horizontally at a given vertical position while being intensity modulated to expose the media to the correct amount light at each location on the line of the media. The particular color being exposed is determined by the vertical position of the electron beam on the face of the CRT. To correctly expose each line of the media to each color, the media, or the CRT, is moved as each color is selected and exposed.
In prior art systems, the image is conducted from the CRT to the media being exposed by a fiber optic array that forms the faceplate of the CRT. The fiber optic array requires that the media be in close proximity during exposure. This can lead to image artifacts due to dirt on the fiber optics or scratching.
Graded Index (GRIN) lenses are known in the art. One embodiment of the GRIN lens is the SelFoc lens manufactured by NSG America, Inc. of Somerset N.J. These lenses are cylindrical, and made of glass. Glass rods are first drawn, and then baked in an oven with chemicals that diffuse into the glass. As the chemicals diffuse into the glass, they reduce the index of refraction of the glass so that the index of refraction varies across the diameter of the lens, causing light passing through the lens to bend around inside the lens.
By comparison, a conventional lens uses the geometry of the surface of the lens to focus the image. That is, the light rays are bent as they pass from air into the lens and also bent as they pass from the lens back into air, because the index of refraction is different between air and the lens material. Within the conventional lens, however, the light rays travel in a straight line since the index of refraction is constant within the conventional lens.
Because the index of refraction varies within the graded index lens, the graded index lens focuses the image internally to the lens. As the image is focused within the lens, it travels in a sinusoidal pattern which causes the image to be oriented in various directions at various locations within the lens. The graded index lens thus has a pitch that determines the orientation of the image. With a lens having a pitch of 0.5, the lens will conduct the image from one end to another and invert the image, as with a conventional lens. With a lens having a pitch of 1, the lens will conduct an image from one end to another and keep the image oriented in the same direction, as would occur with two normal lenses successively coupled.
An advantage of the graded index lens is that it does not have to be bonded to the face of the CRT. A disadvantage of the graded index lens is that it focuses different wavelengths of light (i.e. different colors) at different distances, thus causing chromatic aberration in color images conducted to the photosensitive media. This disadvantage does not affect conventional uses for the lens, such as FAX machines or laser printers, which use monochromatic light to expose the media. However, when used to focus color light, the chromatic aberration of the lens causes a partial defocusing to occur for some colors, depending upon the distance between the image and the media. Thus in the past the use of graded index lenses has been sub-optimal for color printers.
Another advantage of applying a graded index lens to a printer is that the medium does not come into contact or near contact with the surface of the lens array during exposure, thereby greatly reducing the susceptibility to image artifacts due to dirt in the optical path or scratching due to paper contacting the face plate.
There is a need in the art for an apparatus and method for using a graded index lens with color printers. There is a further need for a method and apparatus for overcoming the problem of different colors focusing at different points. The present invention meets these and other needs in the art.
It is an aspect of the present invention to correct the focus of an image being conducted from a face of a CRT through a graded index lens to photosensitive media being exposed by the image.
It is another aspect of the invention to place a face of the CRT at an acute angle to the media being exposed by the image to compensate for chromatic aberration.
Another aspect of the invention is to place the graded index lens at a center of the acute angle, thus the optical distance between the inner CRT face and an inside surface of the lens is equal to the optical distance between the outside surface of the lens and the media being exposed.
A further aspect of the invention is to place at least one color filter between the CRT and the media.
A still further aspect of the invention is to place the color filter on a surface of the CRT.
Still another aspect of the invention is to use a plurality of graded index lenses, one for each color filter.
Yet another aspect of the invention is to compensate for chromatic aberration by placing different materials or thicknesses of materials in the optical paths of the three color channels to adjust the conjugate distance of each color for best focus.
The above and other aspects of the invention are accomplished in a system that optically conducts an image formed on the inner surface of a CRT face plate through a graded index lens to the photosensitive media being exposed. The optical distance between the inner CRT surface (the object) and the Media (the image) is called the Total Conjugate (TC), and different colors focus at different TC distances between the surface and the media. Because of this, if the surface of the CRT is parallel to the media, one or more colors of the image will not focus correctly on the media. The present invention places the face of the CRT at an acute angle with respect to the surface of the media to correct for the TC distance needed for various colors. The graded index lens is placed at a center of the acute angle such that the optical distance between the face of the CRT and a near surface of the graded index lens is equal to the distance between a far surface of the graded index lens and the media, at each location on the surface of the graded index lens.
One embodiment of the present invention uses a different phosphor stripe on the face of the CRT to create each color being exposed. A separate graded index lens array is placed between each phosphor stripe and the media.
A second embodiment uses a CRT having a white phosphor throughout the entire exposure area of the CRT and utilizes color filters to create each color being exposed. The color filters may be placed on a surface of the CRT, on a near surface of the graded index lens, on a far surface of the graded index lens, or adjacent to the media. A separate graded index lens array is placed between each color filter and the media.
Another embodiment uses a single large graded index lens array to span across all phosphor stripes or all color filters.